A continuous caster is used for manufacturing semi-finished products having a predetermined shape by solidifying molten steel while further removing impurities from molten steel, after a secondary refining process. As shown in FIG. 9, the continuous caster comprises a ladle 51, a tundish 52, a mold 53, supporting rolls 54, and a torch cutter (not illustrated) placed at the end of the caster.
If a surface level of molten steel in the mold 53 fluctuates during continuous casting, a cast slab may have a surface defect. Thus, an amount of molten steel discharged from the tundish 52 has to be controlled as constant as possible. A variety of methods have been introduced for controlling molten steel flow, and the most simple and the easiest method herein is an open-pouring method. In the open-pouring method, molten steel is poured into a mold from a nozzle opening of a constant dimension, which is placed at the bottom of the tundish 52, such that molten steel is exposed to the atmosphere (or atmosphere gas) without using a tube such as an immersion nozzle.
Although a tundish nozzle used in this open-pouring method is made of high strength refractories such as zircon series or zirconia series, the tundish nozzle is inevitably affected by chemical and mechanical action from molten steel. As time passes, a nozzle bore becomes larger due to corrosion and accretion grows to cause nozzle clogging, which makes it difficult to maintain a predetermined casting speed. For this reason, a regular replacement of the tundish nozzles is required, and apparatuses enabling the replacement of the tundish nozzles during casting operation have been developed (See Patent Documents 1 and 2, for examples).
As shown in FIG. 9, the tundish nozzle comprises the upper nozzle 57 placed inside of a refractory formed at the bottom of the tundish 52; and the lower nozzle 58 pressed and supported at an undersurface of the upper nozzle 57. A tundish nozzle exchanging device 56 exchanges this lower nozzle 58. Beside the lower nozzle 58 during casting operation, a lower nozzle 59 is placed for next replacement. Now, a hydraulic cylinder 60 pushes the lower nozzle 59 for replacement toward the lower nozzle 58. Then, the lower nozzle 58 during casting operation is displaced from the casting position thereof, and the lower nozzle 59 is set at the casting position.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-286658    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 11-10293